Sputtering method has been heretofore used in forming electrodes or wirings of semiconductor devices. Since the method is advantageous for mass-production, and for assuring safety of the film thereby produced, it has been used where argon ions are forced to collide against refractory metal silicide type target so as to release the metal, and to deposite the same on a substrate opposing to the target in the form of a thin film. Thus, it is apparent that the property of the silicide film formed by sputtering substantially depends upon the property of the target.
However, according to increase in degree of integration and minimization of the integrated semiconductor elements, reduction of particles (minute grains) generated from the sputtering target, in a case of forming a refractory metal silicide thin film, is urgently demanded. The reason of this resides in that the minute particles of approximately from 0.2 to 10 .mu.m generated from the target during the sputtering process tend to be mixed in the deposited thin film, and to cause shortage or disconnection of wiring when the semiconductor device is practically used in a circuit, thus reducing the production yield of the target.
Conventionally, various methods have been proposed for manufacturing targets of high density, and the structure of which is made compact and fine, so that the amount of particles generated from the target is reduced.
For instance, Japanese Patent Laid-Open No. 179534/1986 discloses a method wherein a melted Si is impregnated in a semi-sintered raw material made of a refractory metal component (M) and Si component. According to this method, a structure having spherical or oval shaped MSi.sub.2 of grain diameter 5 to 500 .mu.m dispersed in continuous matrices of Si is obtained, and the amount of contained impurities such as carbon and oxygen is held less than 50 ppm.
On the other hand, Japanese Patent Laid-Open No. 219580/1988 discloses a technique wherein a mixture made of a refractory metal (M) and Si is subjected to a silicide reaction under a high vacuum thereby forming a semi-sintered substance, and this substance is thereafter subjected to a hot isostatic press sintering process for obtaining high density of target. In this case, a compact structure with the maximum grain size of MSi.sub.2 held less than 20 .mu.m and the maximum grain size of free Si held less than 50 .mu.m is obtained. This target has a mixed structure of minute MSi.sub.2 grains and free Si grains dispersed with each other, and the containing amount of Oxygen set less than 200 ppm. Since the Oxygen content of the target is thus suppressed to a low level, the sheet resistance of the resultant thin film can be maintained in a low level.
Furthermore, Japanese Patent Laid-Open Nos. 179061/1988 and 39374/1989 disclose a technique wherein a powdered mixture of a refractory metal (M) and Si is subjected to a silicide reaction under high vacuum, so as to obtain a sintered substance, and this substance is pulverized and added with a composition adjusting silicide powder, and then subjected to a hot-press sintering process so as to obtain a target of high density and Si coagulation suppressed.
However, in the case of the method wherein melted Si is impregnated in a semi-sintered substance, it is found that although a high density target can be obtained as a result of substantial reduction of impurity contents such as carbon, oxygen and the like. The silicon (Si) impregnated in the semi-sintered substance tends to drop out continuously so as to form a matrix; and also that since coarse and large Si portions are formed by Si impregnated in large voids formed in the semi-sintered substance, the Si having a rigidity comparatively smaller than the metal silicide tends to be broken down by the thermal stress caused during the sputtering process, and furthermore since the Si is provided continuously, the strength of the entire target is made insufficient.
Thus, the metal silicide is easily collapsed, and a great number of particles are thereby produced.
In case of the method wherein a semi-sintered substance formed by use of pulverized Si powder as it is for press-sintering, it is also found that although a target of high density and compact structure can obtained, contaminating carbon mixed during the pulverizing process of Si is not removed, but it remains in the target, and therefore during the sputtering operation, sputtering drops are not sputtered sufficiently from a portion containing much carbon, thus causing generation of particles, and furthermore there is a problem that the carbon containing portion formed in the thin film is hardly etched, thus resulting etchant remain and dis-connection of wiring.
In addition, it is also found that in the case where the pulverized Si powder is formed into a semi-sintered substance, and the semi-sintered substance is pulverized, but as it is subjected to press-sintering although a target of high density and compact structure can be obtained, carbon adversely mixed during the pulverizing step of Si is not removed, but is remained in the target. As a consequence, there arises further problem that sputtering drops are not sufficiently sputtered at a portion containing large amount of carbon, and furthermore, a portion of the thin film containing carbon is hardly etched, thus causing presence of residual etchant, and disconnection of wiring.
In the case where pulverized Si powder is formed into a semi-sintered substance, thus obtained semi-sintered substance is again pulverized, a composition adjusting silicide power is added thereto, and the entirety is subjected to a hot-press sintering process; it is also found that although a target of high density and fine structure can be obtained, not only contamination of the material due to carbon increases, but also the content oxygen mixed into the material increases because of the two crashing steps, Accordingly, the generated amount of particles increases, and the electric resistance of the thin film increases because of oxygen mixed in the thin film.
Even in the case of high density target having a density ratio of 99%, it is also confirmed that the generated amount of particles tends to increase under the effect of a specific impurity, and waste products are liable to increase rapidly at a time when a wiring pattern is formed by etching on the thin film.
Heretofore, in view of easiness of control of the composition of the silicide film, a sputtering target manufactured according to a powder-sintering method has been ordinarily used. More specifically, the conventional metal silicide target has been produced by a method wherein a metal silicide (hereinafter noted MSi.sub.2) obtained by reaction Synthesizing metal powder (M) of tungsten, molybdenum and else with silicon power (Si) is subjected together with Si to hot-press or hot isostatic press (Japanese Patent Laid-Open Nos. 141673/1986, 141674/1986, and 178474/1986 and else) or a method wherein Si is impregnated into a silicide semi-sintered substance (Japanese Patent Laid-Open No. 58866/1986).
However, in the case of the former method, since the sintered substance is provided by adding Si powder to synthesized MSi.sub.2 powder, in a case of sintered substance of, for instance, a composition containing MSi.sub.2.2 - MSi.sub.2 the occupied volume ratio of Si phase is held in a range of 8% to 25%, much smaller than that of the MSi.sub.2 phase. Accordingly, in order to sufficiently distribute the Si phase around MSi.sub.2 grains of angular shape obtained by pulverizing, a procedure depending on the press sintering is not sufficient, and a target having defective and nonuniform structure such as including coagulated portion of angular MSi.sub.2 grains and localized portion of Si phase is thereby obtained.
On the other hand, melting point of the MSi.sub.2 phase is much different depending on the kind of the metal M. For instance, the melting points of WSi.sub.2, MoSi.sub.2, TiSi.sub.2 and TaSi.sub.2 are 2165.degree. C., 2030.degree. C., 1540.degree. C. and 2200.degree. C., respectively. In a case where an MSi.sub.2 having melting point thus differing in a wide range and the Si phase of a melting point of 1414.degree. C. are press-sintered at a temperature lower than the eutectic temperature, sintering does not progress between MSi.sub.2 grains of thermally stable, thus reducing the combining strength between grains substantially, and rendering the products to be brittle. Further the remaining pores render the compactness of the structure to be insufficient.
When a silicide film is formed by sputtering utilizing the thus obtained target, the combination between grains tends to be broken by irradiating energy of argon ions, and particles are generated from the sputtering surface of the target due to breakage and collapse starting from the aforementioned defective portions.
Particularly in a case of a high density integrated circuit and the like, the width of electrodes and the spacing between wirings are minimized in accordance with an increase in degree of integration from 4 Mega to 16 Mega, and therefore, the particles mixed in the deposited thin film deteriorates the yield of production rapidly.
In the case of the latter-mentioned conventional method, the composition of the target is controlled by impregnated melted Si in the silicide semi-sintered substance which has been before hand controlled to a predetermined density. However, in a case where MSi.sub.2 is synthesized by silicide reaction between the M powder and Si powder for obtaining a semi-sintered substance of a predetermined density, or where a semi-sintered substance or a predetermined desntiy is formed by the sintering process of press-formed MSi.sub.2, the density is varied depending on the treating temperature and time and the pressing pressure, so that it is extremely difficult to obtain a target of a desired composition.
According to the knowledge of the inventor of this invention, since the powdered MSi.sub.2 and Si to be used as materials are of high purity, there is no tendency of impurities being collected by diffusion in the boundary between MSi.sub.2 phase and Si phase of the target, and therefore the interface bonding strengths between the MSi.sub.2 phase and the Si phase, and between different MSi.sub.2 phases are made weak.
In addition, there is problem that the sputtering operation becomes unstable, because the difference in electric resistance between the MSi.sub.2 phase and Si phase is extremely large. More specifically, the electric resistances of WSi.sub.2, MoSi.sub.2, TiSi.sub.2, TaSi.sub.2, constituting the MSi.sub.2 are 70, 100, 16 and 45 .mu..OMEGA..cm of comparatively small values, respectively, while the electric resistance of the Si phase is extremely large value of 3.times.10.sup.10 .mu..OMEGA..cm. Further, there is no interface layer between the MSi.sub.2 phase and the Si phase, so that the electric resistance in the boundary portion changes abruptly. Particularly in the structure of the target manufactured in accordance with the latter method, it is held in a state that Si phase of high resistance is directly in contact with the MSi.sub.2 phase of low resistance.
Accordingly, when sputtering is carried out by use of such target, insulation break-down between the MSi.sub.2 phase and the Si phase inevitably occurs under a voltage larger than a predetermined value, and an electric current starts to flow abruptly. That is, when the voltage becomes more than a predetermined value, discharge of electricity occurs, and MSi.sub.2 grains of weak interface bonding strength or parts made of Si phase are liable to be collapsed, thus generating the particles.
This invention is made in view of the above described difficulties of prior art, and the object of the invention is to provide a sputtering target of high quality capable of substantially preventing generation of particles, and capable of forming a thin layer of high quality. Another object is to provide a method for manufacturing such a sputtering target.